Disposable personal care products, such as pantiliners, diapers, and tampons, are a great convenience, as are disposable medical care products, such as drapes, gowns, head coverings, and face masks. These products provide the benefit and convenience of one time, sanitary use. However, disposal of many of these products is a concern due to limited landfill space. Incineration of such products is not desirable because of increasing concerns about air quality and because of the costs and difficulty associated with separating these products from other disposed articles that cannot be incinerated. Consequently, there is a need for disposable products which may be quickly and conveniently disposed of without dumping or incineration.
It has been proposed to dispose of such products in municipal and private sewage systems. Ideally, the products would be degradable in conventional sewage systems. Products suited for disposal in sewage systems which can be flushed down conventional toilets and are dispersed or disintegrated in water are termed "flushable." Disposal in this manner is simple, convenient, and sanitary.
Personal care and medical care products must have sufficient strength to maintain integrity under the environmental conditions in which they will be used. They must also be able to withstand the elevated temperature and humidity conditions encountered during use and storage and still lose integrity upon contact with water in the toilet. Therefore, a water-disintegrable material which is capable of thermal processing into a thin film having mechanical integrity is desirable.
Currently, thin films are typically made from water-insoluble polymers or polymer blends. Frequently used polymers include amorphous polymers, epoxy resins, and semicrystalline polymers. Examples of amorphous polymers are polystyrene (PS), styrene-acrylonitrile copolymers, polycarbonate, and poly(vinyl chloride) (PVC). Examples of semicrystalline polymers are polyethylene (PE), polyamide (PA), polybutadiene (PB), and polypropylene (PP). The most commonly used polymers are polypropylene, and polyethylene.
The thin films composed of these polymers are formed by extrusion casting or melt blowing processes. Conventional film extrusion involves mixing commercially available pellets of the desired polymers at increased temperatures, followed by extruding the mixture in a single screw extruder through a slit die to form a film. The film is then cooled by passing it through a series of chilled rolls. Films made in this manner from such water-insoluble polymers are unsuitable for use in "flushable" personal care and medical care products because they do not possess the desired characteristics, e.g., they will not degrade in conventional sewage systems and consequently form blockage in the sewer lines.
Polyethylene oxide (hereinafter PEO) is a hydrophilic, water-soluble polymer,
--(CH.sub.2 CH.sub.2 O).sub.n --
that is produced from the ring opening polymerization of ethylene oxide, ##STR1##
It is available in widely varying molecular weights in the form of a powder from a number of sources, for example, Union Carbide Corp. (Danbury, Conn.) PEO is currently used as a flocculant to enhance the deposition of colloidal particles onto wood pulp fiber in the paper-making process. It is also used as an additive to modify such properties as the aggregation state, sedimentation behavior, and rheology of polymers employed as paints and adhesives. PEO is also used to modify and stabilize polymer lattices, for example, by grafting PEO chains to a polystyrene lattice.
Due to its unique interaction with water and body fluids, the present inventors are considering it as a component material for flushable and personal care products. However, currently available PEO resins are not practical for the formation of thin films by melt extrusion or for personal care product applications for a number of reasons.
For example, while low molecular weight PEO resins have desirable melt viscosity and melt pressure properties for extrusion processing, they have low melt strength and low melt elasticity which limit their ability to be drawn into films having a thickness of less than about 2 mil. Films produced from low molecular weight PEO also have low tensile strength, low ductility, and are too brittle for commercial use.
High molecular weight PEO resins, on the other hand, should produce films having improved mechanical properties compared to those produced from low molecular weight PEO. High molecular weight PEO, however, has poor processibility and poor melt drawability due to its high melt viscosity. Melt pressure and melt temperature must be significantly elevated during melt extrusion of high molecular weight PEO, resulting in PEO degradation and severe melt fracture. Therefore, only very thick films of about 7 mil or greater in thickness can be made from high molecular weight PEO. Films this thick are not practical for flushable applications.
Attempts to melt extrude PEO often result in severe degradation of the PEO. Even when a film can be formed, the PEO undergoes morphological changes such as crystallization and aging, when it is cooled from the melt and exposed to the ambient environment. These changes affect the mechanical properties of the film, resulting in a film that is weak and brittle, having very low elongation-at-break and tear resistance, and, thus, not suitable for the production of personal care products. What is needed in the art, therefore, is a means to overcome the difficulties in melt processing of PEO resins and to improve the resultant ductility and toughness of the thin films formed therefrom.
It is known in the art to modify water-insoluble polymer resins, such as polystyrene and polypropylene, by incorporating soft rubber particles into the polymeric structure to improve the toughness of the polymer, to reduce its modulus, and to improve the softness and flexibility of the resulting material. The modifier can be a rubber-like elastomer, a core-shell modifier, or another polymer, such as styrene butadiene polymers and acrylic polymers. Incorporation of the modifier can significantly reduce the elastic modulus of the polymer under tension. It can also initiate energy dissipation processes in the polymeric structure during deformation resulting in increased elongation at break, enhanced toughness, and improved tear resistance. The efficiency of the modifier depends upon the specific base polymer/modifier composition, blend morphology, phase structure, and toughening mechanisms and process conditions.
The modifier can be incorporated into the base polymer by several different processes. One such process is conventional melt blending methods. These methods involve the blending of a base polymer blend with thermoplastic elastomers or particulate rubbers. The highly dispersive and distributive mixing required is generally achieved with twin screw extruders or with high shear mixers under high temperature and high shear conditions. Another such process is to mix liquid rubber with a monomer of the desired base polymer followed by polymerization of the mixture under conditions that result in a controlled rubber-phase separation.
There are two types of base polymer/modifier systems which can be formed: a dispersed system and a network system. In a dispersed system, the base polymer is a matrix throughout which the modifier particles are dispersed. In a network system, the base polymer is present in the form of particles or islands which are surrounded by thin elastomer layers of the modifier to form a honeycomb-like network. Both types of systems exhibit very fine, well-dispersed morphologies.
Dispersed systems typically exhibit two toughening mechanisms which provide additional energy absorption in the polymer under tension. One mechanism is the preferred formation of crazes at the rubber particles, i.e., stress bearing microcracks with the stretched polymer fibrils. This type of energy absorption is observed in high-impact polystyrene and many grades of acrylonitrile-butadiene-styrene (ABS) polymers. Another mechanism is shear deformation between the modifier particles, i.e., multiple shearing. This type of energy absorption is observed in impact-modified polyamide and polypropylene.
Network or honeycomb systems exhibit a third mechanism of energy absorption in addition to the two mechanisms exhibited by dispersed systems. In this third mechanism there is an intensive yielding of the thermoplastic particles inside the meshes of the network, i.e., multiple particle yielding.
PEO cannot be efficiently modified by the prior art methods described above. It is difficult to melt process PEO under the conditions required to incorporate the rubbery modifiers of the prior art into the PEO matrix because PEO is very sensitive to high shear and high temperature. Attempts to modify PEO in this manner result in poor thermal stability and high shear-induced degradation. Additionally, conventional melt blend extruders employ a series of water baths to cool the resulting polymer strands. Because PEO is water-soluble and water-absorbing, the strands cannot be cooled in this manner.
Polymerization of the PEO monomer/modifier mixture would require the development of expensive and complex steps. Control of the morphology of the resulting blend would be significantly limited, and the success of such a process is unpredictable.
Thus, there is a need in the art for disposable medical and personal care products that will maintain strength and integrity during use and will degrade in conventional sewage systems. Further, there is a need in the art for a process of modifying PEO to improve its melt processibility and mechanical properties.
Additionally, there is a need in the art for tougher, softer, and more tear resistant PEO. There is also a need for a PEO resin having improved melt processing properties. Further, there is a need in the art for a PEO resin which is useful for the production of flushable films, dispersible thin-films, and flushable breathable films.